It has been proposed previously to provide, in a vehicle such as a car, a seat belt pre-tensioner which is an arrangement adapted to apply a positive tension to the seat belt in the event that an accident arises. Such a pre-tensioner comprises means to apply tension to the seat belt, or to a fitting associated with the seat belt, and means to initiate operation of the device in response to an accident occurring.
FR 2 282 303 discloses a pre-tensioner which is connected to a buckle into which a tongue mounted on the seat belt is connected. The pre-tensioner comprises a piston-and-cylinder arrangement, the cylinder containing an explosive charge which is triggered electrically in response to an accident being detected, for example in response to severe deceleration of the vehicle. When the explosive charge is ignited the piston is caused to move up the cylinder, thus effectively applying tension to the safety belt. In alternative examples present in this Specification the explosive charge is used to draw one end of a wire or cable into the cylinder, the other end of the wire or cable being wound on a drum formed on a shaft of a retractor reel. The arrangement is such that as the cable is drawn into the cylinder, the shaft of the retractor reel is caused to rotate, thus applying tension to the safety belt.
In a further arrangement disclosed in this French Specification 2 382 903 the plunger is mounted within the cylinder, and is biassed in one direction by means of a mechanical force-storing device constituted by a spring. However, the plunger is retained in an initial position by means of an over-dead-centre link which serves to lock the plunger. A solenoid is provided adapted to move the over-dead-centre link through the dead-centre position, thus effectively freeing the plunger and enabling the plunger to move under the spring bias applied thereto.
All the arrangements disclosed in the French Specification have their disadvantages. Each of them requires an electric signal to be generated in response to an accident occurring, this electrical signal either triggering an explosive, or operating a solenoid to move a linkage through a dead-centre position. It takes a period of time, albeit a brief period of time, for an electrical signal to trigger an explosive and for an electrical signal to actuate a solenoid. It is to be appreciated that if a seat belt pre-tensioner is to be of value, it is important that the tension is applied to the seat belt as soon as an accident detected. If there is any delay, even if it is a brief delay, the tension may be applied to the seat belt too late to be of any real value.
Various other attempts have been made to provide a seat belt pre-tensioner with an appropriate arrangement to trigger operation of the pre-tensioner, and the present invention seeks to provide an improved arrangement which operates in a satisfactory and reliable manner, but which is relatively economic to fabricate.